tests: Update for infrastructure changes

• tests/scout.sh: Use latest-steam-client-public-beta

  The latest-container-runtime-public-beta alias no longer exists, because the latest container runtime public beta for scout doesn't contain its own copy of scout any more, making the alias misleading. Instead, it's the "layered" runtime previously seen in the scout_layered_slim branch, where we use a Steam Runtime 2 'soldier' container and then layer the scout LD_LIBRARY_PATH runtime included with Steam onto it, for an overall result similar to using the traditional LD_LIBRARY_PATH (non-container) runtime on a Debian 10 system. This provides better compatibility with games that were compiled in a non-pure scout environment, such as Life Is Strange 2.

  The closest equivalent is the latest-steam-client-public-beta alias, which represents the version of the LD_LIBRARY_PATH runtime that is included in the public beta branch of the Steam client, so move over to that when producing non-"layered" scout runtime builds.

• tests: Default to the public download URL for pressure-vessel snapshots

  We now publish these on repo.steampowered.com, so we can hard-code that as a default. This avoids relying on scout builds having their own copy of pressure-vessel included, which the latest beta does not.